Allow writing logs to a file

[flub]

Currently you can only generate logs using the RUST_LOG environment variable. It would be nice if users could easily produce logfiles without any ansi colouring etc. Maybe a --logfile path/to/logfile --loglevel debug. Or some other ways. But this should be easy for users to use.

[robations]

I've just very quickly tried out dumbpipe and, after testing RUST_LOG, my immediate thought was that the logs should go to stderr?

Doing something like this:

dumbpipe listen > data.txt

Creates a mess of data sent plus log messages, which isn't what I was expecting.